Various methods have been developed over the years to dispose of sewage sludge. Such methods include wet spreading, composting, land filling, incineration, and special processing that requires dewatering, drying and heat treatment to produce a fertilizer which is substantially free of pathogens (a product designated by the EPA as “Class A Biosolids”). However, these methods are proving to be controversial and unacceptable in today's environment. As more and more landfills close transportation costs to new locations have increased. Wet application is not sufficiently sanitary, and often results in the contamination of local streams, rivers and groundwater supplies. Although biosolids fertilizer is rich in nutrients, its heavy metal content has raised questions about the acceptability of its repeated application to farmland, which may cause a cumulative buildup of metals like cadmium, lead, mercury, selenium, silver and other toxic metals.
Sewage sludge has a high water content, and requires dewatering prior to wet spreading, composting, land filling, incineration or thermal treatment to manufacture biosolids. Typically, dewatering is carried out with settling tanks, screening systems filter presses, belt filter presses or centrifuges. Drying uses cylindrical rotary kilns, kilns with overlying beds, grinding driers or indirect driers of different construction. A representative apparatus for drying sewerage sludge is disclosed in U.S. Pat. No. 5,634,281. A representative apparatus for incineration of sewage sludge is disclosed in U.S. Pat. No. 3,954,069. Another such apparatus is disclosed in U.S. Pat. No. 4,311,103.
The steps of dewatering and drying sewerage sludge to manufacture biosolids fertilizer requires significant amounts of energy, which is obtained using hydrocarbon-based fuels that are subject to price instability and a growing scarcity. Electrical energy may also be used but is extremely impractical due to its cost.
Ashes from the incineration of sewerage sludge may be disposed of directly into landfills, may receive further processing before disposal, or may be converted into value-added products. U.S. Pat. Nos. 5,079,091 & 4,997,711 teach compaction of sewerage sludge incineration ashes to have the bulk specific gravity of 0.9-2.5 that enables mass dumping without causing secondary pollution. Further, if the compacted ashes are then fired to have the bulk specific gravity of 1.4-3.5, both the volume and weight are reduced much more, realizing easy handling of the resultant burned product. However, due to the high cost of drying and incineration or other processes needed to convert sewerage sludge ash into value-added products, which have yet to be proven cost effective, there is no real recovery of the costs associated with these disposal options.
Thus, there is a need in the art to reduce the cost of processing and disposing of sewerage sludge. This includes the use or disposal of any final by product(s).